This invention relates to a process for forming an ink jet recording element, more particularly to a process for forming an ink jet recording element with a uniform protective overcoat.
In a typical ink jet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof.
An ink jet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-forming layer. Again typically, the ink-receiving layer is either a porous layer that imbibes the ink via capillary action or a polymer layer that swells to absorb the ink.
Ink jet prints, prepared by printing onto ink jet recording elements, are subject to environmental degradation. They are especially vulnerable to water smearing and light fade. For example, since ink jet dyes are water-soluble, they can migrate from their location in the image layer when water comes in contact with the receiver after imaging. Highly swellable hydrophilic layers can take an undesirably long time to dry slowing printing speed, and will dissolve when left in contact with water, destroying printed images. Porous layers speed the absorption of the ink vehicle, but often suffer from insufficient gloss and severe light fade.
In order to reduce the vulnerability of ink jet prints to water damage without resorting to a costly and time-consuming lamination step, receivers with a fusible porous particle topcoat and processes to produce them have been developed. In order to obtain uniform coatings, the coated fluid should have sufficient viscosity or mechanical integrity so that impingement of air during the drying process does not disturb the uniformity of the coated layer. This has generally been accomplished by using hydrophilic polymers that either boost viscosity such as polyvinyl alcohol, or polymers that can be gelled by chilling such as gelatin. There is a problem with this technique, however, in that upon fusing the layer, the hydrophilic binder impairs water resistance.
EP 0 858 905 A1 discloses a process for preparing an ink jet recording element by forming a porous outermost layer by heat treatment of a particulate thermoplastic resin. However, there is a problem with this process in that the coated layer does not set when coated, so that the layer may not always be uniform.
U.S. Pat. No. 5,925,712 discloses a coating composition of a powdered thermoplastic polymer, a water-soluble cationic polymer and a nonionic or cationic latex binder. However, there is a problem with this element in that water-soluble polymers degrade the water resistance of the layer containing them.
It is an object of this invention to provide a process for preparing an ink jet recording element that has an uppermost porous, ink-transporting layer that can be coated from an aqueous solution. It is a further object to provide a process wherein a coated particle dispersion will gel uniformly upon coating prior to the drying stage. Another object is to provide a process for forming a uniform coated layer of particles without the need for a gelling or thickening hydrophilic polymer binder. Still another object is to provide a process for producing a coated porous pass-through layer of thermoplastic particles that will provide a uniformly thick protective layer upon fusing.
These and other objects are achieved in accordance with the invention comprising a method for making a porous ink jet recording element having a uniform, protective layer comprising coating on a support the following layers in order:
a) a porous, image-receiving layer containing a diffusible coagulant, and
b) a layer containing a dispersion of particles which are capable of forming a particle gel with a coagulant;
c) the diffusible coagulant thereby diffusing into the layer containing the dispersion of particles, causing the layer to set to a particle gel; and
d) drying the element to obtain a uniform, protective layer on top of the porous, image-receiving layer.
By use of the invention, a porous, ink jet recording element is obtained that has a uniformly thick protective layer upon fusing.
Particles which are capable of forming a particle gel with a coagulant which may be used in the invention include those which have ionic functionalities on their surfaces, such as ionomer particles, particles which have adsorbed anionic or cationic surfactants, polyelectrolytes, etc. In a preferred embodiment of the invention, the particles are anionomer particles. In yet another preferred embodiment, the anionomer particles are urethane anionomer particles.
Urethane anionomer particles which may be used in the process of the invention can be water-dispersible, segmented polyurethanes having the following formula: 
wherein:
R1 is represented by one or more of the following structures: 
A is derived from a polyol, such as a) a dihydroxy polyester obtained by esterification of a dicarboxylic acid, such as succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic, isophthalic, terephthalic, tetrahydrophthalic acid, and the like, and a diol, such as ethylene glycol, propylene-1,2-glycol, propylene-1,3-glycol, diethylene glycol, butane-1,4-diol, hexane-1,6-diol, octane-1,8-diol, neopentyl glycol, 2-methyl propane-1,3-diol, or the various isomeric bis-hydroxymethylcyclohexanes; b) a dihydroxy polylactone, such as polymers of xcex5-caprolactone and one of the above mentioned diols; c) a dihydroxy polycarbonate obtained, for example, by reacting one of the above-mentioned diols with diaryl carbonates or phosgene; or d) a dihydroxy polyether, such as a polymer or copolymer of styrene oxide, propylene oxide, tetrahydrofuran, butylene oxide or epichlorohydrin;
R2 represents a substituted or unsubstituted alkyl group of from about 1 to about 12 carbon atoms or a substituted or unsubstituted aryl group of from about 6 to about 24 carbon atoms;
R3 represents an alkyl group of from about 1 to about 12 carbon atoms substituted with phosphonate, carboxylate or sulfonate groups, such as dimethylol propionic acid;
w is an integer of from about 10 to about 60 weight %;
x is an integer of from about 20 to about 40 weight %;
y is an integer of from about 50 to about 80 weight %; and
z is an integer of from about 0 to about 10 weight %.
In a preferred embodiment of the invention, the polyurethane has a number average molecular weight of from about 5,000 to about 100,000, more preferably from 10,000 to 50,000. The water-dispersible polyurethane employed in the invention may be prepared as described in xe2x80x9cPolyurethane Handbookxe2x80x9d, Hanser Publishers, Munich Vienna, 1985.
As used herein, a porous, image-receiving layer is one which is usually composed of inorganic or organic particles bonded together by a binder. The amount of particles in this type of coating is often far above the critical particle volume concentration, which results in high porosity in the coating. During the ink jet printing process, ink droplets are rapidly absorbed into the coating through capillary action and the image is dry-to-touch right after it comes out of the printer. Therefore, porous coatings allow a fast xe2x80x9cdryingxe2x80x9d of the ink and produce a smear-resistant image.
In a preferred embodiment of the invention, the porous image-receiving layer comprises from about 20% to about 100% of particles and from about 0% to about 80% of a polymeric binder, preferably from about 80% to about 95% of particles and from about 20% to about 5% of a polymeric binder, such as poly(vinyl alcohol), poly(vinyl pyrrolidinone), poly(vinyl acetate) or copolymers thereof or gelatin. The porous, image-receiving layer can also contain polymer micro-porous structures without inorganic filler particles as shown in U.S. Pat. Nos. 5,374,475 and 4,954,395.
Examples of organic particles which may be used in the image-receiving layer employed in the process of the invention include core/shell particles such as those disclosed in U.S. Ser. No. 09/609/969 of Kapusniak et al., filed Jun. 30, 2000, and homogeneous particles such as those disclosed in U.S. Ser. No. 09/608/466 of Kapusniak et al., filed Jun. 30, 2000, the disclosures of which are hereby incorporated by reference. Examples of organic particles which may be used include acrylic resins, styrenic resins, cellulose derivatives, polyvinyl resins, ethylene-allyl copolymers and polycondensation polymers such as polyesters.
Examples of inorganic particles which may be used in the image-receiving layer employed in the invention include silica, alumina, titanium dioxide, clay, calcium carbonate, barium sulfate, or zinc oxide.
In general, the image-receiving layer may be present in a dry thickness of about 5 to about 60 xcexcm, preferably about 8 to about 45 xcexcm.
As described above, the porous, image-receiving layer contains a diffusible coagulant. A diffusible coagulant may be defined as an electrolyte that will cause the agglomeration in the dispersion of the particles, such as anionomer particles. For clarification of the nomenclature, see xe2x80x9cThe Use of Nomenclature in Dispersion Science and Technologyxe2x80x9d NIST Recommended Practice Guide, Special Publication 960-3 U.S. Department of Commerce, February 2001).
When the concentration of the agglomerates is sufficient, a spanning network of particles, i.e., a particle gel is formed. For example, the urethane anionomer particles used in the process of the invention form an electrostatically stabilized suspension in water by virtue of negatively charged functionality on the particle surface such as a carboxylate. A coagulant neutralizes or suppresses the negative surface charge on the particles leading to agglomeration.
In a preferred embodiment of the invention, the electrolyte is a mineral or organic acid or a salt of a monovalent or multivalent cation. Examples of such mineral acids include hydrochloric acid or sulfuric acid. Examples of organic acids that may be used include toluene sulfonic acid or methanesulfonic acid. Salts of monovalent and multivalent cations include sodium chloride, calcium chloride and aluminum chloride. Additional suitable coagulants and methods to generate them in-situ are described in xe2x80x9cNovel Powder-Processing Methods for Advanced Ceramicsxe2x80x9d J. Am Ceram.Soc.,83 [7], 1557-74 (2000).
After coating a support with the porous, image-receiving layer containing a diffusible coagulant and the layer containing the dispersion of particles which are capable of forming a particle gel with a coagulant, the coagulant diffuses into the image-receiving layer and the layer sets to form a particle gel. The element is then dried for about 5 minutes at a temperature of from about 20xc2x0 C. to about 90xc2x0 C. using conventional drying equipment such a forced air drying.
A uniform top layer is formed since the coagulant diffuses uniformly up to the top layer from the under layer evenly setting the top layer, which upon drying, forms a layer of uniform thickness.
The layers described above may be coated by conventional coating means onto a support material commonly used in this art. Suitable coating methods include, but are not limited to, wound wire rod coating, slot coating, slide hopper coating, gravure, curtain coating and the like. Some of these methods allow for simultaneous coatings of both layers, which is preferred from a manufacturing economic perspective.
If desired in order to improve the adhesion of the image-receiving layer to the support, the surface of the support may be corona discharge-treated prior to applying the image-receiving layer to the support.
The image-receiving and overcoat layers employed in the invention may contain addenda for enhancing its physical and optical properties such as anti-oxidants, surfactants, light stabilizers, anti-static agents, chemical cross-linking agents, cationic mordants and the like.
Any support or substrate may be used in the recording element employed in the invention. There may be used, for example, calendered or uncalendered pulp-based paper, cast coated or clay coated papers, and woven fabrics such as cotton, nylon, polyester, rayon, and the like. In a preferred embodiment of the invention, the support is resin-coated paper. The support usually has a thickness of from about 12 to about 500 xcexcm, preferably from about 75 to 300 xcexcm. Antioxidants, antistatic agents, plasticizers and other known additives may be incorporated into the support, if desired.
Optionally, an additional backing layer or coating may be applied to the backside of a support (i.e., the side of the support opposite the side on which the image-recording layer is coated) for the purposes of improving the machine-handling properties of the recording element, controlling the friction and resistivity thereof, and the like. Typically, the backing layer may comprise a binder and a filler. Typical fillers include amorphous and crystalline silicas, poly(methyl methacrylate), hollow sphere polystyrene beads, micro-crystalline cellulose, zinc oxide, talc, and the like. The filler loaded in the backing layer is generally less than 2 percent by weight of the binder component and the average particle size of the filler material is in the range of 5 to 15 xcexcm, preferably 5 to 10 xcexcm. Typical binders used m the backing layer are polymers such as acrylates, methacrylates, polystyrenes, acrylamides, poly(vinyl chloride)-poly(vinyl acetate) co-polymers, poly(vinyl alcohol), cellulose derivatives, and the like. Additionally, an antistatic agent also can be included in the backing layer to prevent static hindrance of the recording element. Particularly suitable antistatic agents are compounds such as dodecylbenzenesulfonate sodium salt, octylsulfonate potassium salt, oligostyrenesulfonate sodium salt, laurylsulfosuccinate sodium salt, and the like. The antistatic agent may be added to the binder composition in an amount of 0.1 to 15 percent by weight, based on the weight of the binder.
In the present invention, when the ink is ejected from the nozzle of the ink jet printer in the form of individual droplets, the droplets pass through the porous layer containing particles and into the image-receiving layer where most of the dyes in the ink are retained.
Ink jet inks used to image the recording elements of the present invention are well-known in the art. The ink compositions used in ink jet printing typically are liquid compositions comprising a solvent or carrier liquid, dyes or pigments, humectants, organic solvents, detergents, thickeners, preservatives, and the like. The solvent or carrier liquid can be solely water or can be water mixed with other water-miscible solvents such as polyhydric alcohols. Inks in which organic materials such as polyhydric alcohols are the predominant carrier or solvent liquid may also be used. Particularly useful are mixed solvents of water and polyhydric alcohols. The dyes used in such compositions are typically water-soluble direct or acid type dyes. Such liquid compositions have been described extensively in the prior art including, for example, U.S. Pat. Nos. 4,381,946; 4,239,543 and 4,781,758, the disclosures of which are hereby incorporated by reference.
The image-receiving layer used in the recording elements of the present invention can also contain various known additives, such as surfactants such as non-ionic, hydrocarbon or fluorocarbon surfactants or cationic surfactants, such as quaternary ammonium salts for the purpose of improving the aging behavior of the ink-absorbent resin or layer, promoting the absorption and drying of a subsequently applied ink thereto, enhancing the surface uniformity of the ink-receiving layer and adjusting the surface tension of the dried coating; fluorescent dyes; pH controllers; anti-foaming agents; lubricants; preservatives; viscosity modifiers; dye-fixing agents; waterproofing agents, dispersing agents, UV-absorbing agents; mildew-proofing agents; mordants; antistatic agents, anti-oxidants, optical brighteners, and the like. Such additives can be selected from known compounds or materials in accordance with the objects to be achieved.